In chromatography systems, liquids or gases are fed through suitable connection lines between the components of the relevant system. These connection lines, that can be made, for example, from stainless steel, have available at their ends suitable connection systems, also called fittings, in order to be able to create a tight connection with the connections of the components.
Such a connection system was already described in 1975 in U.S. Pat. No. 3,880,452. A capillary tube forming the connection line between two components is here pushed into the capillary tube receptacle opening of a bushing unit or connection unit and fixed in the bushing by means of an attachment screw that has a central borehole for guiding the capillary tube. For sealing, one or more sealing elements that surround the capillary tube in their front end region are pressed into the capillary tube receptacle opening that runs conically inward by means of the attachment screw when the capillary tube and bushing unit are connected.
Here, however, it is disadvantageous that the sealing position is not realized in the plane of the end surface perpendicular to the longitudinal axis of the capillary tube, but is instead offset rearward a certain distance from the end surface in the axial direction. In this way, a dead volume is produced that has a disadvantageous effect, especially in high-performance liquid chromatography. In the case of the extremely high pressures used in high-performance liquid chromatography, in order to be able to guarantee the tightness of such connections, sealing elements are often used like those described, for example, in U.S. Pat. No. 4,619,473 as state of the art (FIG. 2). That publication involves annular sealing elements that are, for the most part, likewise made from stainless steel and that have, in the longitudinal section, a conical profile of the outer diameter. Such a sealing element interacts with a conical receptacle opening in the bushing unit, wherein the conical receptacle opening has a larger angle than the sealing element relative to the longitudinal axis of the capillary tube. Here, when the sealing element is pressed into the receptacle opening, an extremely high, radially inward pressure is exerted by means of an attachment screw onto the front region of the sealing element, so that the sealing position is realized here. Through this pressure, however, for the most part, a deformation of the sealing element and the capillary tube is generated, wherein the sealing element is pressed with its front edge in an annular shape into the outer periphery of the capillary tube. Such a deformation is undesired, especially because the sealing element here is connected to the capillary tube with a positive-fit and non-positive-fit connection, and the sealing element can no longer be easily shifted in the axial direction onto the capillary tube. If the sealing connection is released and should such a plug element be screwed into another bushing unit, for example, because a component of the chromatography system must be replaced, then, indeed, a tight connection can be produced, but, due to tolerances or manufacture-dependent differences in the depth of the receptacle opening, it can no longer be guaranteed that the capillary tube applies a force again with its end surface on the end surface of the line to be connected. If the receptacle opening of the bushing unit of the exchanged component is longer in the axial direction than the previously used component, then an undesired dead volume is created. If the receptacle opening for the capillary tube in the exchanged component is shorter in the axial direction than the previously used component, then the capillary tube is even deformed, possibly damaged, by the pressure of the attachment screw, and a tight connection is no longer possible in certain circumstances. This is because the sealing element mounted on the capillary tube with a positive-fit and non-positive-fit connection cannot move in the axial direction.
However, in the case of such a fitting, a small dead volume also can be rarely avoided, if the end surfaces of the capillary tube and the line to be connected are directly opposite each other or contact each other, because the sealing position is not located in the region of the end surface of the capillary tube or the line to be connected.
In order to be able to compensate for such tolerances or in order to be able to use connection systems of different manufacturers with one and the same plug unit, a self-adjusting plug for high-performance liquid chromatography is described in U.S. Pat. No. 6,494,500 in which the capillary tube is biased axially in the direction toward the capillary tube receptacle openings of the bushing unit by means of a spring provided in the attachment screw. For the seal, an exchangeable ferrule is used that, however, has a conical construction again in its front region and interacts, for forming the seal, with an inner wall of the ferrule receptacle opening that has a more pronounced conical construction. Here, there is the risk again that the ferrule is “crimped tight” onto the capillary tube, especially when the sealing element is made from a metal, for example, stainless steel.
In addition, in this plug unit it is disadvantageous that a support element for the screw spring must be mounted on the capillary tube, wherein this support element makes the production of such a plug unit more complicated.
For avoiding a dead volume, a unit is known from U.S. Pat. No. 4,083,702 for connecting capillary tubes for gas chromatography in which the capillary tubes are similarly connected to each other with their end surfaces abutting. The capillary tubes are fixed by means of annular elements that are wedge-shaped in the longitudinal section and that interact with corresponding conical recesses in the connection housing. In this case, however, the seal is simpler relative to a connection that is compatible with high-performance liquid chromatography, because, in the case of gas chromatography, the pressures that are used are significantly lower, for example, by up to 6 bar. There is no flexibility with respect to the use of the unit for different depths of the capillary tube receptacle borehole.
In addition to the already mentioned disadvantages, the known connection systems feature the disadvantage that the risk arises that the sealing element remains in place in the relevant receptacle opening of the bushing unit when the plug unit is dismounted and the capillary tube is pulled out from the bushing unit.
In addition, the trend in high-performance liquid chromatography is toward using thinner capillary tubes, because these can be more easily bent to better adapt them to the current installation conditions. Due to the production process, thinner capillary tubes also feature smaller tolerances in the inner diameter and smaller eccentricity of the inner diameter. For using capillary tubes with smaller outer diameters in connection with conventional bushing units or connection units, these are provided at the ends with a sleeve for adapting the outer diameter to the diameter of the thicker conventional capillary tubes. However, this represents an additional expense and increases the dead volume of a connection. In addition, in the case of an unfavorable tolerance position, an additional dead volume between the sleeve and capillary tube could also be created.
The present invention is directed to a plug unit for connecting capillary tubes, especially for high-performance liquid chromatography, which can be used in a simple way for capillary tubes with different outer diameters in connection with bushing units with conventional diameters of receptacle openings and here simultaneously offers the possibility of compensating different axial tolerances of receptacle openings in bushing units. In addition, when the plug unit is dismounted, it should be avoided that the sealing element detaches from the capillary tube and remains in the bushing unit. The present invention is also directed to a connection system with such a plug unit.